The following relates to the illumination arts, lighting arts, solid-state lighting arts, and related arts.
Light emitting diode (LED) devices are known for lighting applications. However, a problem with LED devices is that they tend to emit a forward-directed and “peaked” light distribution, for example a Lambertian light distribution, whereas many lamps and lighting applications call for omnidirectional illumination. An ideal omnidirectional illuminator would generate light with precisely the same intensity in every direction over a full sphere. As used in the lighting arts, the term “omnidirectional” illumination encompasses practical approximations to the ideal omnidirectional illumination. By way of illustrative example, some incandescent lamps considered to be omnidirectional light sources provide illumination across the latitude span θ=[0°, 135°] which is uniform to within about ±20% as specified in the proposed Energy Star standard for Integral LED Lamps (2nd draft, May 9, 2009; hereinafter “proposed Energy Star standard”) promulgated by the U.S. Department of Energy. This is generally considered an acceptable illumination distribution uniformity for an omnidirectional lamp, although there is some interest in extending this to a more stringent specification, such as to a latitude span of θ=[0°, 150°] and possibly with a better ±10% uniformity. Such lamps with substantial uniformity over a large latitude range (for example, about θ=[0°, 120°] or more preferably about θ=[0°, 135°] or still more preferably about θ=[0°, 150°]) are generally considered in the art to be omnidirectional lamps, even though the range of uniformity is less than [0°,180°] as would be the case for ideal omnidirectionality.
To construct an LED-based omnidirectional lamp, it is known to employ an array of LED devices mounted on a spherical or otherwise-shaped three-dimensional surface generally centered within an envelope containing a phosphor. Examples of such devices are disclosed, by way of illustrative example, in: Cao, U.S. Pat. No. 6,465,961; Cao, U.S. Pat. No. 6,634,770; Cao, U.S. Pat. No. 6,746,885; Cao, U.S. Pat. No. 7,224,001; Ge, U.S. Pat. No. 7,347,589; and Ge, U.S. Pat. No. 7,497,596. Such devices enable precise tailoring of the light distribution using the shape of the spherical or other LED device mounting surface, the distribution of LED devices on that mounting surface, and the shape and spacing from the mounting surface of the envelope containing phosphor.
However, such devices have certain disadvantages. Manufacturing is complicated since the mounting surface must be formed with the requisite three-dimensional shape and must include printed circuit traces or other wiring for electrically interconnecting the LED devices over this three-dimensional surface. Cost is increased due to the custom-manufactured three-dimensional mounting surface including the requisite printed circuitry or other wiring. Moreover, such lamps employ a substantial number of LED devices, typically of order six or more LED devices. This increases cost as compared with using a fewer number of high power LED devices.
Other known approaches employ a single LED mounted in a spherical envelope including a phosphor. Examples of such devices are disclosed, by way of illustrative example, in: Soules et al., International Publication no. WO 2004/021461 A2 and Eliashevich et al., U.S. Pat. No. 6,661,167. Some embodiments disclosed in this patent employ a single LED device centered in a spherical encapsulant, which may include a phosphor. While such a device can emit omnidirectional illumination, it may be difficult to design the device to meet more stringent omnidirectional light distribution specifications.
Yet another approach is to employ one or more LED devices mounted at a peripheral location or aperture of a spherical envelope containing phosphor. The spherical envelope complements the Lambertian light distribution generated by the LED devices to produce omnidirectional light. Examples of such devices are disclosed, by way of illustrative example, in Bohler et al., U.S. Pub. No. 2007/0267976 A1.
Still yet another approach is to shape the encapsulant to provide more omnidirectional illumination. Examples of such devices are disclosed, by way of illustrative example, in Sommers, U.S. Pat. No. 6,674,096. While such a device can emit omnidirectional illumination, it may be difficult to design the device to meet more stringent omnidirectional light distribution specifications, especially with respect to latitude angles greater than 180°. Further, it is difficult to extend the approach to lamps employing multiple LED devices.